This invention relates to the manufacture of color cathode ray tubes and more particularly to an improved process and apparatus for fabricating the basic structure of a patterned color screen on the viewing panel of a color cathode ray tube.
Cathode ray tubes capable of presenting multi-colored display imagery, such as those employed in color television applications, conventionally utilize face panels having viewing areas whereupon patterned screens are interiorly disposed; such screens being comprised of repetitive groupings of related cathodoluminescent phosphor materials. These screen groupings are normally disposed as bars, stripes, or dots depending upon the type of color tube structure under consideration. For example, in the well-known shadow mask tube construction, the screen pattern is conventionally composed of a vast multitude of similarly-shaped elements such as dots, or elongated areas formed of selected cathodoluminescent phosphors, which, upon predetermined excitation, produce additive primary hues to provide the desired color imagery. The individual phosphor elements comprising the screen pattern are sometimes in substantially tangential contact with one another, while in other instances, they are definitively separated by relatively small interstitial spacings purposely provided to enhance the color purity of the imagery by reducing the possibility of happenstantial electron excitation of adjacent elements. Associated with the screen and spaced therefrom is a foraminated structure or shadowmask which is supported within the face panel by a plurality of mask positioning means embedded in the encompassing sidewall portion of the panel. Each of the apertures of the mask pertains to a specific grouping of related elements of the screen pattern in a manner to enable the selected electron beams traversing the aperture to impinge the proper phosphor element therebeneath.
It has been found that brightness and contrast of the color screen image is markedly enhanced by disposing an opaque light-absorbing material in the interstitial spacing to effect a basic multi-windowed construction for defining the respective elements of the screen pattern. By such structure, each subsequently disposed phosphor element is defined by a substantially dark encompassment which collectively form a patterned window webbing having an array of substantially opaque connecting interstices. During fabrication of this basic web-like structure, one of the process steps involves coating the interior surface of the panel with the opaque material such as, for example, a graphite coating. It is important that this graphite coating be subsequently removed from the sealing edge of the panel and for a fixed distance therefrom up the sidewall of the panel to provide a peripherally defined outline of the basic windowed structure. Any residual graphite adhering to the sealing edge of the panel interferes with the subsequent formation of the hermetic jointure between the panel and the funnel. In addition, since graphite is electrically conductive, any particles thereof remaining on or around the mask supporting studs may be dislodged by movement of the mask locating members and thereby constitute a potential source of deleterious electrical shorts and arcing in the finished tube.
It is a desideratum from both aesthetic and quality considerations, that the basic windowed webbing of the screen structure be framed by a circumscribing opaque peripheral area having a smoothly defined trim line along the upper portion of the panel sidewall in the region proximal to the viewing area. Such definition is desired as subsequent aluminizing of the completed screen should overlay all of the screen area including the opaque periphery therearound. In the completed panel, the remainder of the sidewall below the trim line, including the mask positioning studs oriented therein, and the sealing edge therearound should be free of both graphite and aluminum.
One of the conventional procedures for fabricating the basic windowed webbing portion of the screen structure is comprised of coating the interior of the panel with a negative photo-sensitized material, such as dichromated aqueous-alcohol solution of polyvinyl alcohol. The multi-apertured shadowmask is thence positioned within the coated panel, whereupon exposure radiation emanating from a discretely positioned UV emitter is directed through the multiple openings of the patterned mask to polymerize a similarly-shaped pattern in the sensitized coating therebeneath. In a three-color element screen, such exposure is effected three times, the radiation for each exposure emanating from a differently located radiant energy emitter. Following exposure, the apertured mask member is removed from the panel, and the exposed coating developed to remove the umpolymerized areas, thereby providing a basic polymerized screen format pattern surrounded by a web pattern of substantially bare glass. The whole of the interior of the panel is then overcoated with an opaque colloidal suspension of graphite; whereupon, the interior of the panel is treated with an appropriate degrading agent to effect an effervescent degradation of the polymerized portions of the patterned screen format. This effervescent degradation of the polymerized window areas also loosens the associated graphite overcoating the polymerized material. The loosened materials are then removed by water development to produce an opaque interstitial web having multitudinous windows therein defining bare glass areas wherein the respective phosphor elements of the screen structure are subsequently disposed. Since the aforedescribed degradation affects polymerized areas which are solely related to the viewing area of the panel, the balance of the panel including the sidewall, the mask positioning studs therein, and the sealing edge therearound, remain coated with graphite.
By one conventional procedure, the extraneous graphite coating is removed from the panel edge and a portion of the adjacent sidewall by immersing the sealing edge of the panel into a suitable acid solution which chemically etches away the undesired graphite coating, trimming it to the level determined by the depth of immersion. While this method efficiently removes the graphite from the sealing edge, it manifests two deleterious results. First, by immersing the panel into the acid solution far enough to remove graphite from the mask positioning studs, there is the inherent danger of splashing acid upon the already-formed windowed webbing of the basic screen structure. Any minute droplets of acid splashed upon this critical area destroys the constancy of the pattern, thereby rendering the panel unusable for tube fabrication. To minimize such acid damage, only shallow immersion of the panel sidewall is usually employed, whereupon it is necessary to clean the mask positioning studs by manual techniques, the results of which, are not uniformly consistent. Secondly, the acid immersion process of removing the graphite coating from the panel sidewall often results in the formation of a trim line having a jagged edge. Aside from the undesirable aesthetic appearance of panels trimmed by this procedure, there is a possibility that upon aluminizing, portions of the jagged graphite trim line may exhibit poor adherence and eventually produce loose particles within the tube. Thus, the conventional procedures evidence drawbacks that are deleterious to achieving the desired high quality results.